The lives of aerospace structures, such as aircraft, are limited by fatigue cracking. Fatigue cracks usually begin at holes in the structure. Holes are necessary in order to join together the various components of the structure. Several techniques exist for improving the fatigue life of holes; these techniques often involve the introduction of compressive residual stresses in a circumferential (hoop) direction around the hole. These compressive stresses counteract the tensile stresses imposed by the structure and lead to a lower net tensile stress in the region. Tensile stresses are required to start and propagate a fatigue crack, so the introduction of beneficial residual compressive stresses results in a significant increase in the fatigue life of the structure.
There are various techniques for the introduction of beneficial residual stresses around holes. Some of the common techniques include cold working of the hole before fastener installation, the installation of interference fit fasteners, or the installation of oversqueezed rivets. Cold working before fastening often involves expansion of the hole by passage through the hole of an oversized mandrel, with or without a sleeve to protect the inside diameter. The cold working associated with the passage of the oversize mandrel results in the introduction of beneficial residual compressive stresses. A difficulty with this technique is the inability of the operator to accurately control the amount of cold working, and hence the amount of residual stress. The amount of cold working is determined by the relative sizes of the initial inside diameter of the hole and the outside diameter of the mandrel. Normal machining tolerances on these diameters leads to a variability in the amount of cold work in nominally similar situations. In addition, it is difficult to determine if the proper amount of expansion and residual stress has been introduced unless the initial and final diameters of the holes are accurately measured.
With interference fit fasteners, a fastener whose outside diameter is greater than the inside diameter of the hole is forced into the hole, thereby expanding it and introducing beneficial residual stresses. In this case, it is again difficult to determine if the proper amount of beneficial residual stress has been introduced around the hole.
Likewise, with oversqueezed rivets, the rivet is placed in the hole, then the tail of the rivet is mechanically upset in order to clinch the rivet in place, and an additional amount of force is applied in order to "oversqueeze" the rivet, causing its diameter to increase, expand into the hole, and introduce beneficial residual stresses in the material surrounding the hole. In this operation, it is impossible to determine if the proper amount of "oversqueeze" has been introduced. Current practice usually requires that five rivets be "oversqueezed" in dummy samples before and after actual riveting on the structure. The dummy samples are cut apart and destructively examined to determine if the correct force setting had been applied by the machine. If the sample rivets are good, it is assumed that the rivets in the structure are also good. If they are bad, then all of the intervening rivets must be removed and re-installed. Even if the dummy rivets were good, there is no way to determine if intermittent problems with the machine or variation in the hole or rivet diameters caused a variation in the amount of oversqueeze and hence beneficial residual stress in the actual part. If only one hole of the thousands that exist in a typical structure is improperly prepared, the structure will fail at the defective hole. The weakest link determines the life of the structure.